1. Field of the Invention
The present invention relates to a radiation converting substrate, a radiation image pickup apparatus and a radiation image pickup system, and more particularly to a radiation converting substrate, a radiation image pickup apparatus and a radiation image pickup system including a phosphor layer for converting a radiation into light and a moisture-preventing protective layer.
2. Related Background Art
In the X-ray diagnosis for transmissive inspection of a human body with X-ray, in place for a conventional X-ray photographic method by a sensitizing screen-film system based on a combination of a sensitizing screen and a film, there has been developed and commercialized a digital X-ray image pickup apparatus based on digital radiography, which is a novel radiation image pickup method utilizing a phosphor and a photoelectric converting member. As a radiation-light converting material to be employed in the digital radiography, there is known a phosphor utilizing an alkali halide. Methods for forming a phosphor on the surface of a photoelectric converting device can be principally classified into following two categories:                (1) a method of directly forming a phosphor by solvent coating or vacuum evaporation, either directly or across a protective layer, on a surface of the photoelectric converting device; and        (2) a method of adhering a panel of phosphor with an adhesive material, either directly or across a protective layer, on a surface of the photoelectric converting device.        
Such phosphor based on alkali halide is known to be associated with a drawback that alkali halide in a prolonged storage or a prolonged use causes deliquescence by moisture in the air, thereby resulting in a loss in the resolution of the X-ray image.
In the method (1), for example WO98/36290 discloses, in order to prevent deliquescence of the phosphor material, to cover the phosphor material formed on the surface or the photoelectric converting device or the entire surface of a panel of the photoelectric converting device and the phosphor material with a moisture-preventing protective layer. As a material for the moisture-preventing protective layer, there are disclosed polyparaxylilene, a metallic material and a silicone potting material. On the other hand, Japanese Patent Application Laid-open No. 05-196742 discloses to form a thin film layer between a moisture-preventing barrier layer and a phosphor (scintillator). The thin film layer is provided for improving an adhesion between the moisture-preventing barrier layer and the phosphor (scintillator). As a material for the thin film layer, there are disclosed organic materials such as polyparaxylilene, and organopolysiloxane-polycarbonate.
In the method (2), for example Japanese Patent Application Laid-open No. 2000-356679 discloses, in order to prevent deliquescence of the phosphor material, an invention of covering an entire surface of the phosphor panel with a moisture-preventing protective layer. As a material for the moisture-preventing protective layer, there is disclosed a transparent film such as of paraxylilene.
Patent reference 1: WO98/36290;
Patent reference 2: Japanese Patent Application Laid-open No. 05-196742;
Patent reference 3: Japanese Patent Application Laid-open No. 2000-356679.
A digital X-ray image pickup apparatus formed with the aforementioned method (1) by directly forming a phosphor by solvent coating or vacuum evaporation, either directly or across a protective layer, on the surface of the photoelectric converting device and then forming a moisture-preventing protective layer/a light reflecting layer on the surface of the phosphor, is associated with following drawbacks:
Drawback (A): A polyparaxylilene film cannot provide a sufficient moisture barrier property to alkali halide such CsI:Tl, and CsI shows deliquescence in a prolonged storage.
Such drawback (A) occurs similarly also in a digital X-ray image pickup apparatus formed with the aforementioned method (2) by adhering a panel of phosphor with an adhesive material, either directly or across a protective layer, on the surface of the photoelectric converting device and then forming a moisture-preventing protective layer of paraxylilene at least on a surface of the phosphor of the phosphor panel.
In order to solve the drawback (A), it is conceivable, for increasing the moisture preventing effect of the conventionally known moisture-preventing protective layer, to increase a thickness of paraxylilene film. However, an increase in the thickness of the moisture-preventing protective layer results in a new drawback of a proportional loss in a resolution (CTF) of the X-ray pickup image.
Drawback (B): In case of forming a polyparaxylilene film as a moisture preventing film by thermal CVD on a surface of a phosphor composed of an alkali halide such as CsI:Tl and a light emission activator, there cannot be obtained a sufficient adhesion between a surface of the phosphor constituted of alkali halide and the polyparaxylilene film. On the other hand, in case a moisture-preventing protective layer constituted of a thin metal film is formed on the surface of the polyparaxylilene moisture-preventing film for further improving the moisture preventing effect, an interlayer cleavage occurs at an interface between paraxylilene and alkali halide phosphor because of a stress in the metal film. In a part with a defective adhesion, the function of the moisture-preventing protective layer is deteriorated thereby facilitating the deliquescence of the alkali halide phosphor.